Breathing assistance apparatus

ABSTRACT

A humidifier and humidity sensor is disclosed for use with a breathing assistance apparatus. The humidity sensor preferably includes means to sense absolute humidity, relative humidity and/or temperature at both the patient end and humidifier end. The humidifier may also include provision to both control independently the humidity and temperature of the gases. Further, a chamber manifold is disclosed to facilitate easy connection of the humidifier to various outlets, inlets and sensors. A heated conduit is described which provides a more effective temperature profile along its length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/554,792, filed Oct. 31, 2006 now U.S. Pat. No. 8,235,041, which is adivisional of U.S. patent application Ser. No. 10/686,460, filed on Oct.15, 2003, now U.S. Pat. No. 7,146,979, issued on Dec. 12, 2006, which isa divisional application of U.S. patent application Ser. No. 09/808,567,filed on Mar. 14, 2001, now U.S. Pat. No. 6,918,389, issued on Mar. 3,2005, which claims priority to New Zealand Application No. 503495, filedMar. 21, 2000.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the use of an humidification systemparticularly, but not solely, for providing respiratory assistance topatients receiving mechanical ventilation or respiratory support.

Description of the Related Art

A number of methods are known in the art for supplying humidified gasesto a patient requiring breathing assistance. Such prior art humidifiersgenerally comprise a source of pressurised air (or other mixture ofgases), a humidification chamber including a source of water and aheating means to vaporise the water, and a conduit to convey thehumidified gases to the patient or user.

For example U.S. Pat. No. 4,038,980 describes a “flash vaporisation”humidifier where water drips onto a low thermal mass heater to createrespiratory humidity. It mentions “control means may be providedautomatically to regulate the water supply rate in response to meanssensing the relative humidity”, however they prefer a manual control ofwater flow rate. Thus it incorporates a humidity sensor and controls thewater rate, as opposed to controlling the amount of electrical heating.

U.S. Pat. No. 5,092,326 also describes the use of a humidity sensor in ahumidifier. It describes a high frequency ventilation system thatincorporates a heated humidifier and a humidity sensor, where these arelinked to a central microprocessor. Apparatus is disclosed to moisten agas mixture supplied to the airway, and a microprocessor controls theamount of moisture supplied to the gas mixture. While it discloses ahumidity sensor at the patient airway, it doesn't describe the actualhumidification configuration to be used.

U.S. Pat. No. 5,769,071 describes a humidifier incorporating a heat andmoisture exchanger (HME), supply of water to the HME, heater element andhumidity sensor. The humidity sensor can control humidity via watersupply rate or temperature (via the heater element). Also the humiditysensor is described as being at the patient airway

U.S. Pat. No. 5,988,164 describes a heated breathing tube system for usewith a humidifier. This uses a relative humidity sensor (located nearthe patient) to control the amount of heating provided by the heatedbreathing circuit so that the gas is at a constant level of relativehumidity. The heated breathing circuit may use either electricalheating, or heating via warm recirculating water in a tube. Alsodescribed is a method of control of the electric heater wire or heatedwater tube based on the output of relative humidity sensor.

The previously mentioned U.S. Pat. Nos. 4,038,980 and 5,769,071 bothdescribe humidifiers where the humidification chamber is located close(proximal) to the patient. These have the disadvantage of introducingweight, heat and complexity near the patient which is inconvenient andcould be painful to the patient. Of the cited prior art only U.S. Pat.No. 5,988,164 specifically describes the humidification chamber as beinglocated remotely from the patient.

There are several disadvantages of the prior art systems using ahumidification chamber located remotely from the patient. It is normallyassumed that gases leaving such prior art humidifiers are saturated withwater vapour (100% relative humidity). However there is no guaranteethat the gases leaving such humidifiers are in fact saturated with watervapour. In certain circumstances (e.g. with the incoming air alreadywarm), the gases leaving such humidifiers can be significantly less than100% relative humidity. This is because as they are typically controlledto achieve a desired outlet gas temperature, which in such cases may notbe much more than the incoming air.

Another drawback of the prior art systems is that condensation can occurin the (sometimes heated) conduits connecting the patient to therespiratory assistance equipment. This may occur if the temperatureprofile along such conduits is not even and allows some parts of theconduit to be colder than the gas at these points.

A third disadvantage of such prior art systems is where the gas leavingthe humidifier is at 100% relative humidity it must be heatedimmediately by some form of conduit heater or it may lose heat throughthe walls of the conduit, which results in condensation and therefore adrop in the amount of absolute humidity contained in the gas.

Another fourth disadvantage of the prior art systems is the need for asensor very near to the patient, which adds to the weight and bulk ofequipment at the patient's airway.

A fifth disadvantage of the prior art systems is that intermittent orvarying flow rates will cause the absolute humidity that is generated bythe humidifier to be uneven. This is because the flow rate is varyingfaster than any control loop that might operate in such humidifiers. Airwhich passes through the humidifier at a high flow rate has had littletime to be heated and humidified, while air that passes through thechamber at a low flow rate will be hotter and contain higher absolutehumidity. Consequently it is difficult for a conduit in such prior artsystems to transport these high humidity boluses without condensationand consequent loss of absolute humidity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahumidification system which goes some way to overcoming the abovementioned disadvantages, or which will at least provide the public witha useful choice.

Accordingly in a first aspect the present invention consists in ahumidification apparatus for humidifying gas for a patient or otherperson in need of such gas comprising: an inlet receiving gas, an outletproviding gas with a predetermined humidity and/or temperature, ahumidifier configured to provide water vapour to said gas passingthrough said humidification apparatus, an air heater configured todirectly heat said gas passing through said humidification apparatus inparallel to said humidifier, at least one sensor configured to providean indication of at least two of, relative humidity, absolute humidityand temperature, a controller or processor configured to energise saidhumidifier and said air heater to achieve a predetermined combination ofany two of absolute humidity, relative humidity and temperature.

Wherein said sensor comprises an absolute humidity sensor for providingan indication of the absolute humidity of said gas at least one point inthe flow path through said apparatus of said gas, and said humidifierincluding a body of liquid water.

Wherein said humidifier comprises a metal spiral element to heat saidbody of water.

Wherein said humidifier comprises a heated porous ceramic member adaptedto be in contact with said body of water and said gas.

Wherein humidifier comprises a heated semipermeable membrane adapted tobe in contact with said body of water and said gas.

Wherein said air heater having a humidification bypass, for allowing aportion of said gases to flow to pass from said inlet to said outletsubstantially without humidification.

Wherein said humidification bypass includes a bypass conduit in at leastpartially passing through said body of water for conveying a portion ofsaid gas from said inlet to said outlet, and a valve provided in saidbypass conduit to thereby restrict of the portion of said gas in saidbypass conduit, the gas flowing through said bypass conduit being heatedby the surrounding said body of water.

Wherein said humidification bypass further having a bypass conduit forconveying a portion of said gas from said inlet to said outlet includinga bypass heater adapted to heat the portion of said gas in said bypassconduit and/or said bypass conduit, and a valve provided in said bypassconduit to thereby restrict the portion of said gas in said bypassconduit.

Wherein the restriction provided by said valve on the portion of saidgas in said bypass conduit is in use permanently set.

Wherein the restriction provided by said valve on the portion of saidgas in said bypass conduit is in use manually adjustable.

Wherein a humidification apparatus further comprises a flow sensorproviding an indication of the instantaneous flow rate of wherein saidcontrol configured to control the restriction provided by said valve onthe flow rate of the portion of said gases flow in said bypass conduitbased on said indication of instantaneous flow rate of said gases flowthrough said humidifier, in order that the gases flow exiting from saidhumidifier is of substantially constant humidity.

Wherein said valve comprising an electromechanical actuator connected toa valve member wherein the energisation of said electromechanicalactuator varies the position of said valve member thereby varying therestriction provided by said valve on the flow rate of the portion ofsaid gas in said bypass conduit.

Wherein said valve comprising either a valve member connected to anelastic member or an elastic valve member wherein said valve beingpositioned in said gases flow at said inlet and the position of saidvalve member or said elastic valve member thereby determines the portionof said gas in said bypass conduit.

Wherein the position of said valve member or said elastic valve memberproviding an indication of the rate of flow of said gas at said inlet.

Wherein a humidification apparatus further comprises a conduit to conveysaid gas from said outlet to a patient including insulation adapted tominimise the rate of heat energy lost by said gas in said conduit, saidcontroller adapted to energise said humidifier and said air heater tominimise the condensation of the vapour from said gases in said conduitwhile providing predetermined levels of absolute humidity.

In a second aspect a humidification apparatus for humidifying gas for apatient or other person in need of such gas comprising: an inletreceiving gas, an outlet providing gas with a predetermined humidityand/or temperature, an air heater adjacent to said inlet for heating ofsaid flow of gas, a humidifier configured to provide water vapour tosaid gas passing from said heater to said outlet, in series with saidheater, at least one sensor configured to provide an indication of atleast two of, relative humidity, absolute humidity and temperature, acontroller or processor configured to energise said humidifier and saidair heater to achieve a predetermined combination of any two of absolutehumidity, relative humidity and temperature.

Wherein said air heater comprises a heater wire in a conduit connectedto said inlet.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

The invention consists in the foregoing and also envisages constructionsof which the following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which;

FIG. 1 shows an example of an humidification system, comprised of threeparts,

FIG. 2 shows a chamber which incorporates a metal element,

FIG. 3 shows a chamber using a porous material to provide a heating andhumidifying function,

FIG. 4 shows a chamber using a semipermeable membrane,

FIG. 5 shows a chamber with a variable valve to adjust the ratio of gaswhich are bypassed,

FIG. 6 shows a chamber with an adjustable valve 30 where one part of thegas gets humidified while the other is heated,

FIG. 7 shows a chamber where the dry gas entering chamber is preheated,

FIG. 8 shows a chamber where the dry gas entering chamber is heatedafter leaving the chamber,

FIG. 9 shows a chamber combined with an unheated, well insulateddelivery tube,

FIG. 10 shows construction of a tube incorporating flexible PTC elementsin a parallel wire configuration,

FIG. 11 shows a humidifier configuration using the tube in FIG. 10, and

FIG. 12 shows the chamber manifold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a typical respiratory humidification system,comprised of three parts:

1) a humidification chamber located at a distance from the patient,which heats and substantially saturates gases flowing through it;

2) a delivery system consisting of a flexible tube which carrieshumidified gases from the humidification chamber 1 to the gas outlet 5;and

3) a heater base 3 which heats the humidification chamber 1 and providesmeasurement and control functions.

The gas to be humidified flows into the chamber 1 from port 4 and leavesthe delivery system 2 at gas exit port 5. Gas from exit port 5 flows toa patient via a face mask or similar (not shown). The system iscontrolled using sensors located at positions 7 and 8—typicallytemperature probes. Dry gases at the gas input 4 are heated andhumidified by passing over the surface of hot water 6 in the chamber 1so that they are substantially saturated with water vapour when theyleave chamber 1 at exit port 10. Hot water 6 is heated by heater plate 9and the amount of heating is controlled so that the gas reaches apredetermined temperature at exit port 10. This temperature is measuredby sensor 7. Therefore the humidification chamber 1 acts to heat andhumidify the medical gases so that they are substantially saturated atthe output of chamber 1, and are at a predetermined temperature.

The gas delivery system 2 (also known as a delivery tube or breathingcircuit) consists of a flexible tube 11 containing a heater 12, whichmay consist of a heated resistance wire. The gas from the humidificationchamber 1 passes through the tube 11 and is heated by heater 12 tooffset heat losses through the walls of tube 11. The amount of heatingapplied to heater 12 is regulated so that the gas reaches apredetermined temperature at gas outlet 5, as measured by sensor 8. Thecontrol temperature at sensor 8 is usually higher than the controltemperature at sensor 7, so that the gas is heated along tube 11 toensure that condensation doesn't occur in the tube.

The system as described has gas entering gas inlet 4 from a continuousflow gas source (not shown) and exiting the system through gas outlet 5.However the system is equally applicable where the gas source is aventilator, which creates intermittent flow patterns to provide breathsto a patient. In this case gas outlet port 5 is connected directly togas inlet port 16. The patient is connected to port 17 via anendotracheal tube or similar (not shown). During patient inspiration drygases from the ventilator enter the system at inlet port 4, pass throughchamber 1, delivery system 2, pass through wye-piece 13 and reach thepatient through port 17. During patient exhalation gases pass backthrough port 17, through wye-piece 13, tube 14 and leave through gasoutlet port 18. Tube 14 may also be heated by heater 15 to preventcondensation.

Absolute Humidity Sensing

Humidifiers incorporating humidity sensors for display or control havebeen described in the prior art, however all used humidity sensors whichwere positioned at the patient airway. The current work describes novelhumidifier configurations incorporating a humidity generating chamberlocated at a position which is remote from the patient, a heatedbreathing circuit to transfer humidity to the patient, and humiditysensors to control the level of absolute or relative humidity suppliedto the patient. These humidity sensors are to be located either: 1) atthe chamber outlet only, 2) at both the chamber outlet and near thepatient, or 3) near the patient only.

One aspect of the present invention would be to use a humidity sensor assensor 7. The purpose of humidity sensor 7 is to determine the absoluteamount of humidity which is being generated by chamber 1. Accordingly anabsolute humidity sensor would be ideal for use as sensor 7, althoughthe use of a relative humidity sensor with associated temperature sensorcould equally be used. This system has the advantage of creating acontrolled level of absolute humidity at chamber outlet 10, however thislevel of absolute humidity may not reach the patient if condensation isallowed to occur in tube 11.

An alternative system which would overcome this disadvantage is to use asecond absolute humidity sensor at point 8 instead of a temperaturesensor. The difference in absolute humidity between sensors 7 and 8allows the humidifier to determine whether condensation is occurringbetween the two points. If the two absolute humidity sensors 7 and 8read the same level of absolute humidity then no condensation isoccurring in the tube. If the absolute humidity at sensor 7 is greaterthan at sensor 8, then the difference shows the rate of condensationthat is occurring.

One control strategy would be to control the amount of heating providedto heater 12 so that the absolute humidity difference is reduced tozero. However the tube may still contain mobile condensate because thehumidity difference only describes the rate of condensation, not theabsolute amount of condensate in the tube. Another control strategy isto remove this condensate and hence create a dry tube by heating heater12 so that the rate of measured condensation is negative (i.e.condensation is being evaporated in tube 11) until the measuredcondensation rate reaches zero, indicating that all of the condensatehas been removed. The amount of heating can then be reduced until thesensors show that condensation has just started to occur, then theheating can be increased slightly to the optimum level. Drying out ofthe tube may be a continuous process, or may be initiated at regulartime intervals.

Another variation of the system shown in FIG. 1 would be to use atemperature sensor for sensor 7 and an absolute humidity sensor at point8. This system is simpler than having an absolute humidity at bothpoints 7 and 8. In operation the controller would have to adjust theamount of heating at heater 12 and heater plate 9 so that the correctlevel of absolute humidity was reached without condensate in deliverytube 12. In practice two separate control algorithms would be required,one to control the amount of heating occurring in tube 11 so that nocondensation occurred, and another to control heater plate 9 so that thedesired level of absolute humidity was generated in chamber 1. The twoalgorithms could work concurrently because the heater plate 9 willrespond slower than heater 12, so quick changes in absolute humiditywould indicate the action of heater 12. Sensor 7 provides a controlpoint for heater plate 9, but may not be needed.

Low Relative Humidity Chambers

All systems described so far have used a chamber 1 which attempts tohumidify the gas leaving gas outlet 10 to a high level of relativehumidity. While this condition isn't essential for the correct operationof the new humidification configurations just described because they usehumidity control, it was essential for the prior art humidifier wherecontrol is purely based on temperature. However there are someadvantages to be gained from using a chamber which heats gases to thecorrect absolute humidity, but at a low relative humidity (i.e. thetemperature of the gas is higher than the dewpoint of the gas, thereforethe gas is not saturated).

The first advantage is that it is easier to design a heated deliverysystem to transport such a gas without condensation, since the gasdoesn't need to be heated immediately after it enters the delivery tubeto prevent condensation. Secondly, the use of low relative humiditygases leaving the chamber means that the heater element 12 can be ratedat a lower power than would otherwise be the case, as the gas alreadyhas a higher energy content and can tolerate a greater loss of energybefore the gas condenses in the tube 12. It may even be possible to usean unheated, well insulated breathing circuit instead of a heatedbreathing circuit if the chamber provides gas with enough energy. Notethat low relative humidity chambers can only be used if the heating tothe chamber is controlled using an absolute humidity sensor, not atemperature sensor, since otherwise the absolute humidity output wouldbe too low.

To this end, some humidification chamber configurations which provide ahigh temperature, low relative humidity gas output are shown in FIGS.2-8. FIG. 2 shows a chamber which incorporates a metal element 20 (e.g.a spiral scroll shape), but without wicking paper attached. Thisprovides both dry heating (via the metal element) and heatedhumidification from the heated water 21. With this configuration thechamber 19 provides gas which is not saturated because some of theheating provided to the gas is dry heating via the metal scroll. Therelative humidity generated by the chamber is affected by the gas flowpath, scroll shape, dimensions, and the water level, and so is notreadily adjustable in use. However chamber 19 does give the condensatereducing advantages provided by a low relative humidity, controlledabsolute humidity output.

FIGS. 3 and 4 are alternative humidification chambers which provide lowrelative humidity, high temperature gases at their output. FIG. 3 showsa chamber using a porous material 22 (such as a porous ceramic)containing water 23 to provide a heating and humidifying function, whileFIG. 4 shows a chamber using a semipermeable membrane 24 to provide abarrier to the water 25 in the chamber. In both cases these chambersprovide dry heating via the porous or semipermeable material, as well asheated humidification from the water. In both cases the ratio of heatingto humidifying is fixed and cannot be easily adjusted except by limitingthe water supply.

FIGS. 5 to 8 show chambers that can supply gases at varying levels ofrelative humidity and temperature. In FIG. 5 a variable valve 26 allowsus to adjust the ratio of gas which passes through the dry bypass tube27 to that which flows across the surface of the water 28. The bypasstube passes under the water to heat the gas. The two gas streams mergeat the output 29. This is an example of a “parallel” system where thegas splits and takes two different paths to provide heating andhumidification. In FIG. 6 the gas is again split into two gas pathsusing an adjustable valve 30. One part of the gas gets humidified bypassing across the water 31 in chamber 32, while the other is heated byheater 58, which surrounds tube 33. The gas paths merge at junction 34.

The angle of variable valves 26 and 30 in FIGS. 5 and 6, may bepermanently set, may be manually adjustable 1300, or may beautomatically adjustable for example by electromechanical actuation1400. One advantage of an automatically adjustable valve would be toprovide a constant level of humidity out of the chamber when used withintermittent flow rates, for example when used with a ventilator. Theseflow patterns can be a problem because parts of the breath cycle containless humidity than other parts, due to the chamber providing lesshumidity at higher flow rates. One way to overcome this problem is tomeasure the instantaneous flow rate using a fast response flow sensor,and then rapidly adjusting the angle of the variable valve. A morepractical method of achieving this effect would be to spring-load valves26 and 30 using springs 70 and 71 or use an elastic valve member to formthe variable valve. This would mean that low flow rates would mostlypass through the bypass tubes, while high flow rates would operate thespring-loaded valve and allow more gas to pass across the water in thehumidification chamber. The angle of the spring-loaded variable valvecould also be used by the humidifier to measure the gas flow rate.

FIGS. 7 and 8 show alternative series configurations for low relativehumidity chambers, where the dry gas entering chamber 35 containingheated water 36 is either pre-heated via heater 37 in FIG. 7, or heatedvia heater 38 in FIG. 8 after leaving the chamber. In both cases theheater provides dry heating to the gas and results in a low relativehumidity, high temperature gas leaving outlet 39.

Any of the low relative humidity, high temperature chambers shown inFIGS. 2 to 8 can be used in conjunction with the humidity controlschemes described previously in this patent, but not successfully withthe prior art humidifier due to it being temperature controlled, nothumidity controlled.

Insulated Delivery Tube

Another facet of the invention is shown in FIG. 9. Here the low relativehumidity, high temperature humidification system from FIG. 8 has beencombined with an unheated, well insulated delivery tube. The incominggas enters at port 135 into the standard humidification chamber 35containing water 36 which is heated by heater plate 38. The gas issubstantially saturated in the chamber then leaves the chamber throughgas outlet 39 and enters heated tube section 40 which heats the humidgas to a higher temperature, so that it has a low relative humidity. Thegas then passes through tube 41 which has an insulating layer 42 aroundit. Preferably the insulating layer is a thin jacket of stagnant airwhich reduces heat loss. As the high temperature gas, low relativehumidity gas passes through the insulating tube, a small amount of heatis lost through the tube walls, and therefore the gas cools. However theamount of heating applied to heater 40 is controlled, so that the gas isnever allowed to cool below its dewpoint, which would result incondensation within tube 41.

Several different sensor configurations are proposed. Firstly, sensor 43could be an absolute humidity sensor which controls heater plate 38 sothat chamber 36 produces the desired level of humidity. In oneembodiment sensor 45 is a temperature sensor, which controls heater 40so that the gas passing sensor 45 remains at a certain desiredtemperature. If this temperature is greater than the dewpoint of the gasat sensor 43, then condensation should not occur in tube 41. Howeverthere may already be condensate in tube 41 when the humidifier is turnedon. If a humidity sensor is used for sensor 45 instead of a temperaturesensor, then the level of condensate occurring in the tube 41 can becontrolled. The algorithms described earlier in this patent fordual-humidity sensor control can be used with this system.

An alternative location for the absolute humidity sensor is at position44 instead of 43. The absolute humidity here should be the same as at 43because the gas has been heated and so hasn't lost any moisture. Howeverthere may be advantages to placing the absolute humidity sensor at 44,for instance due to better sensor operation in a low relative humidityenvironment. This location for the absolute humidity sensor can be usedwith either a temperature or absolute humidity sensor at location 45.

Humidifier Configurations Without any Patient Airway Sensors

Yet another aspect of this patent relates to removing the need for asensor at the patient airway. To remove this sensor safely, we must becertain that the gas entering the delivery tube has a safe level oftemperature and absolute humidity, and that the surfaces inside thedelivery tube do not exceed safe temperature levels. This implies adelivery tube that has a constant internal wall temperature.

It would be desirable, therefore, to have a heated delivery tube whichself-regulates its temperature at a desired level. The heater couldeither be embedded in the wall of the delivery tube itself, or it couldlie inside the lumen of the delivery tube, or it could be wrapped aroundthe outside of the delivery tube. Such a heater could be made frompositive temperature coefficient (PTC) material (such as “Winterguard”from Raychem Corp., Menlo Park, Calif. USA), so that the resistance ofthe heater increases if the heater is hot, resulting in reduced power.However the delivery tube may pass through more than one environment, ormay have localised drafts present on certain parts of the tube. If thePTC elements are arranged in parallel, then the full benefit of the PTCheater can be envisaged. If the PTC elements are arranged in parallel,then the cold portions of the tube will have a lower resistance, whichwill result in more heat being dissipated. Thus the tube will tend toregulate its own temperature.

FIG. 10 shows construction of a tube incorporating flexible PTC elementsin a parallel wire configuration. The tube 48 is made of a flexible PTCmaterial, which has two low resistive strip connections, 46 and 47, oneither side of it. This allows each portion of the tube to consist ofshort conducting segments of tube connected in parallel betweenconductors 46 and 47. These segments are represented by dotted linesencircling the tube in FIG. 10. The conductors 46 and 47 are connectedto adjustable voltage source 49, which may be AC or DC. The tube wouldhave an outer layer (not shown) which provides electrical insulation andthermal insulation to the tube. Each longitudinal segment of the tubewill be able to regulate its own temperature independently of the restof the tube. To enhance this operation, it may be necessary to provideparallel slots 50 running perpendicular to the axis of the tube, toeliminate electrical cross-connection between the different PTCsegments.

Although one specific PTC heated tube design has been envisaged anddescribed, other PTC tube designs could be used. It may also be ofadvantage to create a PTC tube that has a differing temperature profilealong its length rather than a constant temperature profile. The PTCdesign could also be extended to incorporate PTC heaters in other partsof the patient breathing circuit, such as the flexible extension tubewhich is usually connected between the Y-piece (port 17 of FIG. 1) andthe patient's endotracheal tube. A further extension of the PTC tubeconcept would be into a self-heated and temperature controlledendotracheal tube.

The PTC tube described in FIG. 10 allows us to create a humidifier whichdoesn't use any sensor at the patient airway. FIG. 11 shows a humidifierconfiguration using this tube. Gas enters humidification chamber 52 viainlet port 51 and is humidified by water 53, heated by heater plate 54.Absolute humidity sensor 55 controls the heater plate so that the gaspassing sensor 55 is at a desired level of absolute humidity. PTC tube56 is heated by an external voltage (not shown) so that the internalsurface temperature is at a constant desired temperature, which isselected to be above the dewpoint of the gas. The gas which leaves tube56 at outlet 57 will therefore be near the temperature of the tube, andcontaining the desired level of absolute humidity which was controlledby absolute humidity sensor 55.

A variation of the system shown in FIG. 11 would be to use a temperaturesensor at position 55. Another variation of a tube with a constantinternal wall temperature would a delivery tube with heated water orother fluid pumped through smaller conduits in the wall of the deliverytube. Since the heated fluid has a high specific heat relative to air,the temperature of the fluid remains fairly constant during passagethrough the delivery wall conduits.

Use of a Sensor/Heater Manifold

Traditional humidifiers have tended to use sensors that are probeshaped, so that they can be inserted through specifically designed holesin the side of the breathing circuit to measure temperature. However thehumidifier configurations that have been described in this patentincorporate many sensors around the chamber, so the use of a manifold 59as shown in FIG. 12 may be useful.

The humidification chamber 60 is a removable item which can be slid ontothe humidifier base 61 as shown in FIG. 12. As the chamber 60 is slidonto the humidifier base 61, its base makes contact with heater plate 62and its inlet and outlet ports 63 and 64 make contact with holes 67 and68 inside the manifold 59. Dry air to be humidified enters the manifoldat port 65, passes out of the manifold through port 67, and flowsthrough port 63 into the chamber 60, where it is humidified.

After leaving chamber 60 the humid gas passes through chamber port 64into manifold port 68. Finally the humid gas leaves manifold 59 throughport 66 and passes to the breathing circuit.

The manifold may be a separate, removable assembly, or it may be anintegral part of the humidifier base. It may contain temperaturesensors, humidity sensors, flow sensors, or a heater element. Thesewould be located inside the manifold 59 at positions 72 and 73. Themanifold 59 may be heated to prevent condensation of humid gas. It couldconnect to both chamber ports 63 and 64 as described, or it may onlyconnect to the outlet port 64. One advantage of using a manifold is thatmany sensors or heaters can be combined in a single, cleanable assembly,rather than requiring separate probes which need to be plugged into thebreathing circuit. This simplifies connection and setup for the user.Another advantage of a manifold is that the incoming dry gas temperatureand flow rate can easily be measured without additional probes andconnections.

Variations on the Described Configurations

Although absolute humidity sensors have been described with all of thedifferent humidification schemes described in this patent, relativehumidity sensors could also be used. This may involve slightly differentcontrol algorithms to the ones described in this patent. Alternatively,a relative humidity sensor could be combined with a temperature sensor.This allows the absolute humidity to be calculated from relativehumidity and temperature, rather than being measured directly.

All of the novel humidification schemes that have been described in thispatent could be used with additional temperature sensors. These mayprovide additional benefits such as providing a safety backup in theevent of a failed humidity sensor. Another benefit would be maintainingthe temperature being delivered to the patient within certain limits sothat the relative humidity is not too low, even though the absolutehumidity was acceptable.

Similarly it may be useful to measure the air flowrate through thehumidifier, as this is an important parameter which affects humidifiercontrol. Therefore flow sensors could be incorporated within any of thepreviously described systems. One useful prior art flow sensorconstruction would be to use a sensor based on heat loss from a hotelement in the airstream. If a heated humidity sensor is used, theamount of heating that is required for the sensor to achieve temperaturecan be used to determine the gas flow rate.

Infection control is a prime consideration when designing medicalcomponents. To prevent bacterial colonisation of the components in thehumidification system, any parts which come in contact with the gasstream could be made out of antibacterial plastic. To preventcontamination of sensor probes, the probe ports could incorporate adisposable sheath which protects the probe from pathogens in thebreathing circuit. This would be particularly applicable to temperatureprobes. In general humidity probes need to have contact with the gasstream so a disposable sheath would be inapplicable to humidity sensors,unless they worked on optical principles, or unless the sheath was madeof water vapour permeable material, which did not allow the passage ofpathogens. The protective sheath could be an integral part of adisposable breathing circuit.

What is claimed is:
 1. A humidification apparatus for humidifying a gases flow to be supplied to a patient or other person comprising: a humidifier base comprising a heater plate; a humidification chamber having an inlet and an outlet to allow the gases flow to pass through the humidification chamber from the inlet to the outlet, the humidification chamber being configured to be mounted on the humidifier base in contact with the heater plate; a removable assembly that connects the humidification chamber to the humidifier base, the removable assembly forming a portion of the humidification apparatus and including a first sensor being supported by the removable assembly, the first sensor being configured to determine a first parameter of the gases flow before the gases flow enters the humidification chamber; and a controller that controls the heater plate based upon at least the first parameter to heat water in the humidification chamber to humidify the gases flow passing through the humidification chamber.
 2. The apparatus of claim 1, wherein the first sensor is located adjacent to the inlet of the humidification chamber when the humidification chamber is mounted on the humidifier base.
 3. The apparatus of claim 2, futher comprising a continuous flow gases source.
 4. The apparatus of claim 3, wherein the continuous flow gases source is a medical gases supply.
 5. The apparatus of claim 1, wherein the first sensor is located upstream of the inlet of the humidification chamber when the humidification chamber is mounted on the humidifier base.
 6. The apparatus of claim 1, further comprising a second sensor configured to determine a second parameter of the gases flow.
 7. The apparatus of claim 6, wherein the controller is configured to control the heater plate also based on the second parameter.
 8. The apparatus of claim 6, wherein at least one of the first and second sensors is a humidity sensor.
 9. The apparatus of claim 8, wherein at least one of the first and second sensors is a temperature sensor.
 10. The apparatus of claim 1, wherein the removable assembly comprises a manifold.
 11. The apparatus of claim 1, wherein the removable assembly comprises at least one gases passageway.
 12. A humidification apparatus configured to humidify a flow of gases for delivery to a user, the apparatus comprising: a flow path of the flow of gases extending from a gases source to a humidification chamber; the humidification chamber defining a volume configured to contain a volume of liquid; a heater base comprising a heater configured to heat the volume of liquid, the heater being controlled by a controller; and a removable assembly that connects the humidification chamber to the heater base, a first sensor mounted to the removable assembly, the first sensor being positioned to sense a property of gases flow before the humidification chamber; the first sensor being connected to the controller such that the controller is configured to control the heater that is configured to heat the volume of liquid contained within the humidification chamber by adjusting a temperature of the volume of liquid in the humidification chamber based upon output from the first sensor.
 13. The apparatus of claim 12, wherein the first sensor is positioned between the gases source and the humidification chamber.
 14. The apparatus of claim 12, wherein the first sensor is configured to sense a humidity level of the flow of gases.
 15. The apparatus of claim 12, wherein the first sensor is configured to sense a temperature of the flow of gases.
 16. The apparatus of claim 12, wherein the removable assembly comprises a manifold.
 17. The apparatus of claim 12, wherein the removable assembly comprises at least one gases passageway.
 18. A method for humidifying a gases flow to be supplied to a patient or other person comprising: sensing, using a sensor carried by a removable assembly that connects a humidification chamber to a heater base, the sensor being located on the removable assembly upstream of the humidification chamber, the sensor configured to sense a first parameter of the gases flow inside of the removable assembly prior to the gases flow entering the humidification chamber; controlling a heater thermally coupled to a volume of liquid in the humidification chamber using output from the sensor located on the removable assembly upstream of the humidification chamber; transferring water vapor from the volume of liquid to the gases flow passing through the humidification chamber; and conveying the gases flow out of the removable assembly after the gases flow exits the humidification chamber, wherein the first parameter is selected from the group consisting of temperature, absolute humidity, relative humidity and flow rate and is used to control the heater in the generation of the water vapor.
 19. The method of claim 18, further comprising sensing a second parameter of the gases flow after the gases flow exits the humidification chamber.
 20. The method of claim 19, wherein one or more first sensors senses the first parameter and one or more second sensors senses the second parameter, the one or more first sensors being located before the humidification chamber along a flow path of the gases flow through the removable assembly and the one or more second sensors being located after the humidification chamber along a flow path of the gases flow through the removable assembly. 